Thermal inkjet hardcopy devices such as printers, graphics plotters, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Color inkjet hardcopy devices commonly employ a plurality of print cartridges, usually two to four, mounted in the printer carriage to produce a full spectrum of colors. In a printer with four cartridges, each print cartridge can contain a different color ink, with the commonly used base colors being cyan, magenta, yellow, and black. In a printer with two cartridges, one cartridge can contain black ink with the other cartridge being a tri-compartment cartridge containing the base color cyan, magenta and yellow inks, or alternatively, two dual-compartment cartridges may be used to contain the four color inks. In addition, two tri-compartment cartridges may be used to contain six base color inks, for example, black, cyan, magenta, yellow, light cyan and light magenta. Further, other combinations can be employed depending on the number of different base color inks to be used.
The base colors are produced on the media by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same dot location, with the overprinting of two or more base colors producing the secondary colors according to well established optical principles.
For many applications, such as personal computer printers and fax machines, the ink reservoir has been incorporated into the pen body such that when the pen runs out of ink, the entire pen, including the printhead, is replaced.
However, for other hardcopy applications, such as large format plotting of engineering drawings, color posters and the like, there is a requirement for the use of much larger volumes of ink than can be contained within the replaceable pens. Therefore, various off-board ink reservoir systems have been developed recently which provide an external stationary ink supply connected to the scanning cartridge via a tube. The external ink supply is typically known as an "off-axis," "off-board," or "off-carriage" ink supply. While providing increased ink capacity, these off-carriage systems also present a number of problems.
These various problems include accurate and inexpensive methods of alignment of the multiple print cartridges, providing a reliable fluid seal between the print cartridge and the external ink supply and providing electrical connections between the print cartridge and the printer.
When using a printhead with a large number of nozzles and high resolution, correct alignment of all the nozzles so that the ink is correctly placed on the print media is extremely important. Dot alignment must be done in both the horizontal and vertical axes. The horizontal axis is generally referred to as the scan axis. The vertical axis is generally referred to as the paper advance axis. Many factors can affect alignment. For example, slight variations during manufacturing can affect alignment. Additionally, the alignment of ink placed on a page can be dynamically affected during printing, for example dependent on the volume of a drop which is generated by merged droplets.
In color printing, the various colored dots produced by each of the print cartridges are selectively overlapped to create crisp images composed of virtually any color of the visible spectrum. To create a single dot on paper having a color which requires a blend of two or more of the colors provided by different print cartridges, the nozzle plates on each of the cartridges must be precisely aligned so that a dot ejected from a selected nozzle in one cartridge overlaps a dot ejected from a corresponding nozzle in another cartridge. This requires each of the nozzle plates on the print cartridges to be aligned with respect to one another within a few tens of microns after being installed in the carriage.
One method of accomplishing this is to make the print cartridge bodies identical to one another so that, when multiple print cartridges were installed in the carriage, the print cartridge bodies were all aligned with one another in the carriage irrespective of any misalignment of the nozzle plates secured to the print cartridge bodies. To align the nozzle plate on a print cartridge so that nozzle plates are positioned in ideally the same location on all the various print cartridges, the nozzle plate is typically glued in position on the print cartridge relative to a molded-in plastic datum formed on the print cartridge body. This alignment process has a significant drawback in that the glue curing process causes the nozzle plate to slightly shift as the glue is being cured. In addition, molded-in stresses in the plastic cartridge body creep during the thermal curing process. Since this movement is substantially unpredictable, this alignment and gluing process can only produce print cartridges of minimal accuracy.
Other, more expensive techniques have been used to achieve higher alignment precision. One of these techniques automatically detects any misalignment of the nozzle plates once the print cartridges have been installed in a carriage and then mechanically adjusts the positions of the print cartridges in the carriage. Using another relatively expensive method, an ink drop detector within the ink printer measures the location of a drop of ejected ink after being ejected from a nozzle, and a software algorithm compensates for any misalignment of the nozzle plates. Both of these techniques significantly increase the cost of the ink printer.
Achieving accurate mechanical alignment between printheads using in-printer latching is very difficult and costly. Moreover, typically the desired alignment is not achieved and further ways of adjusting are used. For example, the user can print a test plot with the four pens, and then type in a code that sets an alignment algorithm in the printer. This is inconvenient and users rarely do this and print quality suffers.
Multichamber printheads which use a single substrate, typically a single piece of silicon, with multiple ink feed slots help solve the above problems, but there are still trajectory and drop volume variations within a multichamber cartridge. In addition, the multichamber printhead requires that all nozzles be on the same substrate, thus, upper limits on silicon die size limit the performance of multichamber cartridge systems.
Thus, there is needed is an accurate method of assuring precise dot-to-dot registration between multiple colorants in an inkjet printer.